Analysis of an Intense Winter Extratropical Cyclone that Advanced along the South Coast of Japan

Abstract

A winter extratropical cyclone underwent rapid development along the south coast and east of Japan on 12 February 1994. The evolution and structure of this cyclone were investigated from its formation through the rapid developing stage using subjective surface re-analyses, objective analyses for upper levels, satellite images and other data. The cyclone formed over eastern China within a broad low-level baroclinic zone. A cold air mass covered the East China Sea and around Japan behind a preceding cyclone, while strong southerly winds brought warm, moist air above and near the incipient cyclone. The surface cyclone progressed eastward over the East China Sea with a moderate deepening rate. It then began developing rapidly south of Japan, over the warm Kuroshio Current. A deep mid-tropospheric trough approached the surface low from northwest and the cyclone extended horizontally as well as vertically. A subtropical jet streak and an associated upper-tropospheric PV anomaly advanced from the rear of the surface low. This cyclone had a cloud system of about 2000 km scale. Active cloud areas were found mostly to the north and east sides of the surface low.
The incipient cyclone lacked well-defined surface fronts. As it advanced over the East China Sea, frontogenesis proceeded rapidly to form a warm front with appreciable temperature gradient, which extended zonally from the cyclone center. A low-level cold air flow, corresponding to a cold conveyer belt, occupied the northern side of the cyclone. Cold frontgenesis in terms of thermal fields was inactive to the rear of the cyclone center. A strong wind shear line extended in a north-south direction from the low center, between the southerly winds to the east and the westerly winds to the west. The shear line was associated with weak temperature gradient. As the cyclone developed the warm front extended westward relative to the cyclone center to form a bent-back front. The bent-back front was accompanied by significant wind shear between the northeasterlies to the north and westerlies to the south. The frontal system of the developing cyclone thus exhibited a characteristic configuration that differs from the classical conceptual cyclone model.